"Zombie" or active? An alternative explanation to the properties of star-forming galaxies at high redshift

TL;DR

This paper proposes that weak AGN activity can explain the unusual infrared and ultraviolet properties of high-redshift star-forming galaxies, challenging previous models that required extreme assumptions.

Contribution

It introduces an alternative explanation attributing the spectral anomalies to hidden AGN activity, supported by spectral and X-ray property comparisons with known AGN types.

Findings

High-redshift galaxies show spectral similarities to AGN.

Expected X-ray fluxes are consistent with moderate AGN activity.

A weak AGN contribution can account for observed spectral properties.

Abstract

Star-forming galaxies at high redshift show anomalous values of infrared excess, which can be described only by extremizing the existing relations between the shape of their ultraviolet continuum emission and their infrared-to-ultraviolet luminosity ratio, or by constructing \textit{ad-hoc} models of star formation and dust distribution. We present an alternative explanation, based on unveiled AGN activity, to the existence of such galaxies. In fact, the presence of a weak AGN configures as a natural scenario in order to explain the observed spectral properties of such high-$z$ objects in terms of a continuum slope distribution rather than altered infrared excesses, due to the different shape of the AGN continuum emission with respect to quiescent galaxies. To this aim, we directly compare the infrared-to-ultraviolet properties of high-redshift galaxies to those of known categories of AGN (quasars and Seyferts). We also infer the characteristics of their possible X-ray emission. We find a strong similarity between the spectral shapes and luminosity ratios of AGN with the corresponding properties of such galaxies. In addition, we derive expected X-ray fluxes that are compatible with energetics from AGN activity. We conclude that a moderate AGN contribution to the UV emission of such high-$z$ objects is a valid alternative to explain their spectral properties. Even the presence of an active nucleus in each source would not violate the expected quasar statistics. Furthermore, we suggest that the observed similarities between anomalous star-forming galaxies and quasars may provide a benchmark for future theoretical and observational studies on the galaxy population in the early Universe.